1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing tyres for vehicle wheels.
2. Description of the Related Art
Generally, in tyre manufacture for vehicles wheels, subsequently to a step of building a green tyre through assembling of respective components of elastomeric material some of which are provided with suitable reinforcing structures, a moulding and curing treatment is provided to be carried out to the aim of causing structural stabilisation of the tyre through cross-linking of the elastomeric components of same as well as, as generally required, of impressing a desired tread pattern into the tyre itself, and possible distinctive graphic marks at the tyre sidewalls.
To this aim, the green tyre is introduced into a suitably heated vulcanisation mould, having a moulding cavity conforming in shape to the final conformation to be given to the tyre itself. After carrying out closure of the mould, the green tyre is pressed against the holding walls of the moulding cavity and the heat required for tyre vulcanisation is simultaneously transmitted thereto. To reach this purpose, for example, expansion of a bladder of toroidal conformation is caused within the tyre through admission of steam under pressure into said bladder, so as to bring the bladder into contact with the inner surface of the tyre and compress the latter against the holding walls of the moulding cavity.
When vulcanisation has been completed, the mould is opened to enable removal of the tyre and arrange the mould for a new moulding/vulcanisation cycle.
Once the green tyre has been introduced into the mould, in order to prevent air pockets or pockets of other fluid possibly used in the vulcanisation process from being entrapped between the holding walls of the moulding cavity and the outer surface of the green tyre, the moulds of the conventional type are provided with a plurality of venting ducts passing through the mould wall and opening into the moulding cavity, said ducts lying at right angles to the holding walls of said moulding cavity. These ducts during the step of pressing the tyre against the holding walls of the moulding cavity, enable evacuation of possible air pockets and ensure a perfect adhesion of the outer surface of the green tyre to the holding walls of the moulding cavity. Adoption of ducts passing through and opening into the moulding cavity however, involves a plurality of problems due to seeping of the green-tyre blend into the ducts themselves during the moulding step. In fact, the blend entrapped in the ducts is subjected to vulcanisation and forms a plurality of threadlike outgrowths on the finished tyre that are known in the specific technical field as “flashes” or “burrs” and are generally removed in a work station dedicated to this purpose, when the tyre production process has been completed. Alternatively, these threadlike outgrowths are not removed from the outer surface of the finished tyre and in this case they involve worsening of the aesthetic-qualitative level of said tyre.
The state of the art proposes many solutions aiming at reducing formation of these threadlike outgrowths.
For example, in document CA-2,190,720 use of a movable valve is provided that is inserted in each of the holes formed in the mould walls, said valve comprising a stem and a closing head associated with said stem. The closing head has a frustoconical shape provided with a flat surface facing the inside of the moulding cavity. A return spring pushes the valve to the open position at which the closing head lies spaced apart from a respective seat of a mating shape, designed to receive said stem and closing head. The valve is forced to the closed position by the thrust exerted thereon by the tyre blend during the moulding step and comes back to the open position when the tyre is removed from the mould.
Document GB-2,339,163 discloses a venting device comprising a closing element slidably disposed within a main body of said device. A first and a second springs enable movement of the closing element—relative to the main body—so as to allow opening and closing of the device. At the inner surface of the moulding cavity, the main body of the device is provided with a cone-shaped seat. At the upper end of the closing element there is a closing surface of conical shape, matching the shape of the cone-shaped seat of the main body of the device. Closure of the valve takes place when the cone-shaped seat and closing surface are engaged with each other through movement of the closing element within the main body.
Document EP-1,380,397 discloses a venting apparatus positioned in a vent of a mould. Said apparatus comprises: a venting duct; a valve body inserted in said duct; a spring surrounding a lower portion of the valve body to supply said valve body with an elastic force; and a retainer in engagement with a lower end portion of the valve body to control vertical displacement of the valve itself. The inner diameter of the venting duct gradually increases towards the moulding cavity to define a tapered shape. In addition, the valve body has a frustoconical closing head provided with an inclined lower surface the inclination of which is different from the inclination of the inner wall of the venting duct. In this manner, when the closing head comes into contact with the inner surface of the duct, contact does not take place along a surface but along a circumference because in radial section the contact between the lower surface of the closing head and the inner surface of the duct is of the punctual type.
The Applicant has noticed that, in the open position, the lower tapered portion of the closing head of the known art valves (the above described valves, for example) faces the inside of the moulding cavity and greatly projects relative to the inner surface of said cavity, this surface being defined by the holding walls of said moulding cavity. This aspect is particularly disadvantageous because in some regions of the mould, in particular at the mould regions designed to form the tyre shoulders, the blend tends to flow tangential to the mould wall and to approach the valve head in a lateral direction relative to said valves. In this way the blend exerts a side thrust on the lower tapered portion of the closing head, this thrust generating a force directed to move the closing head away from the valve seat and to open the valve itself instead of closing it. Therefore, this force counteracts the thrust exerted by the blend approaching the closing head in a direction perpendicular to the inner wall of the moulding cavity and aimed at carrying out the valve closure.
The Applicant has further noticed that the presence of blend interposed between the closing head and the seat of the valve compromises a correct closure of the valve itself and brings to formation of faults on the finished product. Repetition of this phenomenon during subsequent vulcanisation cycles further involves an increasingly greater storage of cured blend within the valve, so that, cycle after cycle, an unavoidable stoppage of the valve itself will be caused.
Finally, the Applicant has ascertained that the side thrust of the blend on the closing head of the valve can cause misalignment of the valve stem relative to the duct in which the valve slides. Since a passage necessary to enable the air to flow out of the moulding cavity is defined between the stem and duct, the amount of the stem inclination due to the above described side force can be of such a value that sliding of the valve within the valve seat and a correct air flow are impaired.